The invention relates to a system and a method for reducing the deposition rate of volatile contaminants onto an optical element of a substrate processing system.
Substrate processing systems are used to fabricate semiconductor logic and memory devices, flat panel displays, CD ROMs, and other devices. During processing, such substrates may be subjected to chemical vapor deposition (CVD) and rapid thermal processes (RTP). RTP processes include, for example, such rapid thermal annealing (RTA), rapid thermal cleaning (RTC), rapid thermal CVD (RTCVD), rapid thermal oxidation (RTO), and rapid thermal nitridation (RTN). RTP systems usually include a heating element formed from one or more lamps which radiatively heat the substrate through a light-transmissive window; RTP systems may also include one or more other optical elements, such as an optically reflective surface facing the backside of the substrate and one or more optical detectors for measuring the temperature of the substrate during processing.
Layers of doped glass, such as borophosphosilicate glass (BPSG) or phosphosilicate glass (PSG), are used extensively in pre-metal dielectric (PMD) layers in logic and memory devices. Doped glass layers are typically deposited onto a substrate in a CVD system and are subsequently heated to a high temperature in an RTP chamber or a furnace. In one heating process, doped glass is densified by heating the doped glass to a temperature of 700-800.degree. C. in an RTP chamber. Heating the doped glass in this way reduces the porosity of the layer, relieves stress in the film, drives off residual impurities left from CVD deposition, stabilizes the dopants against atmospheric instability, and activates the gettering capability of the phosphorous oxides (PO.sub.x) in the film for trapping alkali ions. BPSG can be heated to higher temperatures, such as 850-950.degree. C., to decrease the viscosity of the BPSG and cause macroscopically visible flow (reflow) that planarizes the BPSG surface and enables the BPSG to fill surface features of underlying layers.
Various volatile contaminants are produced during an RTP process as a result of heating a substrate to a high temperatures. These volatile contaminants can condense onto the walls and other surfaces inside the processing chamber and, over time, the build-up of such deposits may detrimentally impact the operation of the processing system. For example, boron oxides (BO.sub.x) and PO.sub.x have high vapor pressures and are produced at a rapid rate when BPSG and PSG layers are heated to high temperatures. To reduce the detrimental impact of these volatile contaminants upon the fabrication process, such RTP systems must be periodically shutdown and cleaned. Replaceable liners (and the like) have been developed for reducing the time needed to clean the processing chamber.